1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device capable of storing binary or trinary or more data in a memory cell and to an IC memory card using the same.
2. Description of the Related Art
In a nonvolatile semiconductor memory device such as a flash memory, a binary type memory cell structure for storing data taking two values of "b 0" and "1" in one memory cell transistor is usual.
Along with the recent demands for increasing the capacity of semiconductor memory devices, a so-called multi-level type nonvolatile semiconductor memory device for storing trinary or more data in one memory cell transistor has been proposed (refer to for example "A Multi-Level 32 Mb Memory" '95 ISSCC P132 on).
A nonvolatile semiconductor memory device capable of storing multi-level data, including binary data, in this way is generally used as a so-called data storage.
As data storage use flash memories, large capacity memories such as 64 Mbit NAND type flash memories have recently started to appear. Along with this, the market for large capacity products such as digital still cameras is now becoming more active.
Further, an integrated circuit (IC) memory card using a flash memory as a storage medium has been proposed and put onto the market and is being used as part of digital still cameras.
While flash memory cards have been used for the storing of still images in digital still cameras in recent years, its use is expected to spread also to recording of music and storing of moving pictures in the future.
An increase of the capacity of a flash memory card in principle requires an increase of the capacity of a flash memory chip, but an increase of the capacity of a flash memory can also be realized by achieving making the memory cells multi-level for storing a plurality of data (bits) in one memory cell in addition to scaling of the semiconductor.
In general, making memory cells multi-level increases the memory capacity, but also causes a reduction in the write speed and read speed and a reduction in the guaranteed number of repeated rewrites.
Namely, in a flash memory card using such a multi-level flash memory, there is a reduction in the write and read speeds and a reduction in the guaranteed number of repeated rewrites compared with a flash memory card using a non-multi-level flash memory.
In information apparatuses such as portable PCs and portable data terminals, the number of repeated rewrites into the flash memory card used in these apparatuses is large, so use of a flash memory card mounting a non-multi-level flash memory is most suitable.
The guaranteed number of repeated rewrites is about 100,000.
However, in general consumer apparatuses, in particular audio video (AV) apparatuses for storing stationary images, music, and moving pictures, the number of repeated rewrites of the data into the flash memory card is small so it is expected that flash memory cards mounting multi-level flash memories will be used.
The guaranteed number of repeated rewrites is less than 10,000 in this case.
Note however that the storage capacity of the card becomes larger, that is, the unit price of a byte becomes lower, in a flash memory card mounting a multi-level flash memory in comparison with a flash memory card mounting a non-multi-level flash memory.
As described above, therefore, a flash memory card mounting a multi-level flash memory is well suited for use for general consumer electronics.
In this way, it is projected that different types of flash memory cards will be used for different applications, that is, one type for products requiring flash memory cards having a fast write and read speed and a large guaranteed number of repeated rewrites and another type for products requiring flash memory cards having a slow write and read speed and a small number of repeated rewrites.
Accordingly, users will have to take the trouble of selecting which product he or she needs limited depending on his or her purpose.
Further, it will be necessary in order to develop specialized flash memory cards, specialized flash memory chips, and specialized controllers for controlling the same for each type of flash memory.